In such discs the recording of the information is carried out using the well known technique of signal modulation known as NRZI (non-return to zero, in which a bit at logic 1 level is associated and represented by a signal transition and a 0 bit is associated with an absence of transition) and particular code formats, belonging to the family of DC-free codes and having limited non-zero runlength.
"Runlength" RL is understood to mean the distance, expressed as a number of binary symbols or bits, between bits associated with a signal transition. In the case of the NRZI technique it is therefore understood to mean the number of binary symbols which have the same logic 0 value and are placed consecutively in a sequence and which separate individual 1 bits.
The 1 bits are necessarily separated from one another since, otherwise, we would have RL=0.
The runlength limitation is represented by the expression RLL(d,k) where d and k respectively represent the minimum and maximum number of symbol units having logic 0 value.
For the coding formats used in optical discs, d is equal to 2 and k equal to 10.
Even if the runlength is limited to the same extent (RLL(2,10)), the format used in compact discs CDs known by the acronym EFM is substantially different from the EFM-PLUS format used in digital video discs DVDs.
With the EFM format an 8-bit binary information item (byte) is converted into a 14-bit code word to which are appended 3 "MERGING" bits in order to ensure compliance with the runlength limitation even when concatenating code words, whatever the following code word is, and also to limit the DC component of the signal resulting from the reading of the code word and of the associated merging bits.
With the EFM-PLUS format a byte is converted into a 16-bit code word.
To ensure compliance with the runlength limitation when concatenating the code words, the coding has to take into account what the following code word will be.
The code words associated with a particular byte configuration are therefore in general different in both cases.
Furthermore, whereas in the case of the EFM format the various byte configurations are each associated in a one-to-one manner with a particular code word, in the case of the EFM-PLUS format the correspondence is not one-to-one and in general a plurality of code words (synonyms) may be associated with each byte configuration, and the same code word may be associated, depending on the concatenation, with different byte configurations, i.e. on the original information, giving rise to ambiguity.
In either case, not all the code word configurations which may be represented respectively by from 14 to 16 bits (2.sup.14 and 2.sup.16 of them respectively) are used, but only those which comply with the runlength limitation.
This has weighty consequences in the decoding phase.
This can occur only via extremely complex logic arrays, via associative memories for transcoding with a high number of cells, or via random access read-only memories (ROM) with considerable capacity respectively of 2.sup.14 addresses and at least 2.sup.17 addresses respectively.
Indeed, in the case of the EFM-PLUS format it is necessary to resolve possible ambiguities (aliasing) with the use of additional information (at least one bit) derived from analyses of the code word immediately following that which is to be decoded.
Therefore, the decoding function, in order to be carried out, requires a considerable number of logic elements, difficult to integrate into a single integrated circuit, on which they thus occupy appreciable space.
Finally, the functions for decoding the two formats have to be carried out by completely separate logic circuits, this increasing the complexity thereof and the congestion within an integrated circuit and consequently the cost.